This invention relates to a stabilised access method and apparatus for use between two relatively moving structures to enable work to be performed or personnel to be transferred as well as equipment. In a particular preferred embodiment this invention is concerned with a stabilised working or transfer platform or capsule for personnel and is primarily intended for ship-borne use whereby the platform or capsule carried by a water-borne vessel remains substantially stationary in relation to a fixed structure which is to be accessed by the platform or capsule.
It has been proposed to provide a work platform mounted on an articulated arm assembly to provide access to a moving structure. In EP 0067669 such a device uses a clamp to connect the platform with the moving structure once it has been positioned. The device is intended to be used mounted on a fixed structure, such as a wharf, for work to be performed on a moving structure, such as a ship. This disclosure mentions a motion sensing means to effect adjustment of the platform but discards the idea as being too complicated, expensive and possibly unsafe.
It is one of the objects of this invention to provide an apparatus, preferably to be ship-borne, and by which a platform or transport capsule for personnel may be positioned relative to a fixed structure and thereafter spatially stabilised to avoid, or at least substantially reduce, relative motion.
Another object is to provide a method for control and stabilisation of an apparatus for such a purpose.
In accordance with one aspect of this invention there is provided an apparatus for providing access between a moving structure and a fixed structure, the apparatus comprising a base mounted on one structure, a support arm assembly mounted on the base with motional freedom in one or more planes or directions relative to the base, a carrier (such as a platform or support structure) mounted on the support arm, first means for control of the support arm to spatially position the support structure and carrier to a selected location and second means to subsequently effect adjustment of the support assembly to maintain the spatial position of the carrier at the selected location. In a preferred embodiment the support arm is pivotally mounted on the base, advantageously using a gimbal mount, and supports an articulated boom supporting the carrier, being a work platform or capsule, which may also be articulated to the boom. The support arm, boom and carrier are controlled, in a first phase by the first means to appropriately position the platform and in a second phase by the second means to compensate for movements of the base.
This invention also provides a method for the control of an access assembly including a carrier supported by a boom or arm and relatively adjustable with respect to a mounting to position the carrier to a selected location, characterised by a first control means to move and position the carrier spatially and a second control means to provide compensation movements to maintain the spatial position of the carrier following displacement of the mounting.
The first and second means may comprise hydraulic systems using independent actuators operating on a relevant pivot joint in parallel. The first actuator serving to position and counterbalance the assembly and hold a position with the second means providing compensating adjustment. The first means may include accumulators to permit movement relative to the held position. Thus positioning is effected by coarse control which can tolerate slow movement over large displacements, whilst compensation is effected by a fine control giving rapid response.
The system may also have only one actuator if both hydraulic control systems, that is the static positioning and dynamic compensation systems, are fed to the same actuator.
Hydraulic control is preferred and the means used may comprise actuators of various types such as piston and ram assemblies, rotary actuators hydraulic motors. Other systems such as electric actuators and drive means may be used.
The sensing system for maintaining the spatial position may comprise gyros and accelerometers, but optical, radio, electronic or mechanical systems referenced to a fixed or known location such as the sea-bed or the structure being accessed may be used as well as positional error corrected satellite positioning systems.